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- Thread starter dijkarte
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- #1

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- #2

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It turns out it doesn't matter in the end how you define current direction (the signs will cancel). It is arbitrary (since it is an unknown) and the equations all work out as long as you are CONSISTENT. I would follow the method that makes more sense to you. For kicks, you can try solving a simple circuit using both methods and you will see they agree.

- #3

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x2. They are both the same thing. If you try a very simple circuit that requires nodal analysis you will notice that they both turn out the same at the end.It turns out it doesn't matter in the end how you define current direction (the signs will cancel). It is arbitrary (since it is an unknown) and the equations all work out as long as you are CONSISTENT. I would follow the method that makes more sense to you. For kicks, you can try solving a simple circuit using both methods and you will see they agree.

- #4

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And when doing nodal analysis, since current direction consistency does not matter, what about element polarity? Can I assume different ones when working on a different node?

Thanks.

- #5

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First

For this circuit from KCL

(Va - 4V)/6Ω = (10V - Va)/10Ω + (6V - Va)/4Ω

And after solve this I get

Va -> 190/31 = 6.12903226V

Second

And again form KCL

Or

So know all current flow outward (away) from the node.

So this assume that voltage at node Va should be at higher potential.

And current flow from + to - , I assume that if current entering into a node I give him " +"

and current that come out form the node I give "-".

But you can choose whatever you wont but you must be consistent in your choice

So I can write:

And again the answer is exactly the same

Va -> 190/31 = 6.12903226V

And maybe another example for this diagram I choose

But I also can assume that if current flow out from the node I give a "+" sign.

I1 + I2 + I3 = 0 ( no current entering the node).

Since all current flow out from the node the Va node should be at higher potential.

And the answer is

Va -> 190/31 = 6.12903226V

- #6

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After reading your example, I tried a different one and this time using at each node different directions for the same current. It worked like a charm! the trick is consistency in applying the sign convention.

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- #8

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Wait till you get to Johns and Martin to see how people REALLY design circuits.

- #9

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Nilsson and Riedel!

- #10

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Can you post the full title of this book. Because Goggle don't help much.while it's nothing compared to Sudoki & Alexander?

And also here you can find a very good video lecture about circuit theory.

http://www.digilentinc.com/Classroom/RealAnalog/index.cfm

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I find it the most comprehensive electricity book compared to many others, in terms of context and presentation.

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